Expression of heterologous DNA sequences in a plant host is dependent upon the presence of an operably-linked promoter that is functional within the plant host. Choice of the promoter sequence will determine when and where within the plant the heterologous DNA sequence is expressed. Where continuous expression is desired throughout the cells of a plant, constitutive promoters are utilized. In contrast, where gene expression in response to a stimulus is desired, inducible promoters are the regulatory elements of choice. Where expression in specific tissues or organs is desired, tissue-preferred promoters are used. That is, these promoters can drive expression in specific tissues or organs. Additional regulatory sequences upstream and/or downstream from the core promoter sequence can be included in expression cassettes of transformation vectors to bring about varying levels of expression of heterologous nucleotide sequences in a transgenic plant. See, for example, U.S. Pat. No. 5,850,018.
Regulatory sequences may also be useful in controlling temporal and/or spatial expression of endogenous DNA. For example, specialized tissues are involved in fertilization and seed development. Identification of promoters which are active in these seed tissues is of interest.
In grain crops of agronomic importance, seed formation is the ultimate goal of plant development. Seeds are harvested for use in food, feed, and industrial products. The quantities and proportions of protein, oil, and starch components in those seeds determine their utility and value.
The timing of seed development is critical. Environmental conditions at any point prior to fertilization through seed maturation may affect the quality and quantity of seed produced. In particular, the first 10 to 12 days after pollination (the lag phase) are critical in maize seed development. Several developmental events during the lag phase are important determinants of the fate of subsequent seed growth and development. (Cheikh, et al., (1994) Plant Physiology 106:45-51) Therefore, a means to influence plant development, particularly in response to stress during this phase of growth, is of interest. Identification of a promoter sequence active in tissues of developing seeds exposed to abiotic stresses would be useful.
Specialized plant tissues are central to seed development. Following fertilization, developing seeds become sinks for carbon translocated via the phloem from sites of photosynthesis. However, developing cereal seeds have no direct vascular connections with the plant; instead, a short-distance transport mechanism operates to move the assimilates from the vascular tissues to the endosperm and embryo. For example, in maize, photosynthate enters the seed via the pedicel; in wheat, via the nucellar projection and the aleurone layer. It is possible that this short-distance assimilate pathway between the phloem and the endosperm can operate to regulate the rate of sucrose transport into the grain. (Bewley and Black. Seeds: Physiology of Development and Germination. N.Y., Plenum Press, 1985. pp. 38-39). Therefore, a promoter active in gene expression within these specialized tissues, such as the pedicel, may have significant effects on grain development.
During rapid seed growth, sucrose is unloaded passively from the phloem into the apoplast of the pedicel parenchyma and inverted to hexose sugars by a cell-wall-bound acid invertase. The hydrolysis of sucrose in the apoplast maintains a favorable gradient for continued unloading from the phloem and provides hexoses that are taken up by the basal endosperm cells. It has been shown that seeds induced to abort, in vitro, have only low levels of invertase activity in the pedicel. (Hanft, et al., (1986) Plant Physiol. 81:503-510)
Water stress to the plant around anthesis often results in seed abortion or restricted development. Studies suggest that sucrose continues to unload from the phloem at low ovary water potential, but it accumulates in the symplasm and apoplasm of the pedicel because of low invertase activity. (Zinselmeier, et al., (1995) Plant Physiol. 107:385-391) This conclusion is supported by the findings of Miller and Chourey (Plant Cell 4:297-305 (1992)), who showed that developmental failure of miniature-1 seeds of maize was linked to lack of invertase activity in the pedicel tissue during the early stages of seed development.
Other specialized plant tissues are also closely involved in the critical processes of fertilization and seed development. For example, in maize, the carpels, which make up the ovary wall, become the pericarp, a tough, protective outer seed covering. The scutellum, along with the endosperm, is involved in translocation of assimilates to the developing embryo. The aleurone, the surface layer of endosperm cells, develops to serve as a source of enzymes necessary in germination. (Kiesselbach, The Structure and Reproduction of Corn. N.Y., Cold Spring Harbor Press, 1999)
To achieve yield stability in high-density plantings, under drought conditions, or in other adverse environments, modification of carbohydrate metabolism during early ear and kernel development may be desirable. Effective control of genes involved in carbohydrate metabolism is dependent on identification and use of a promoter with high levels of tissue and temporal specificity. Specifically desired expression targets include pedicel, pericarp, and nucellus tissue during a period 14 days before pollination to 14 days after pollination.
In light of the important contributions of these specialized seed tissues to proper grain development, identification of a promoter sequence affecting gene expression in these tissues would be useful. Further, it would be desirable to identify a promoter sequence active in these specific tissues at appropriate, critical times. Even more desirable would be the identification of a promoter sequence active in these specific tissues at appropriate, critical times, which is not negatively affected by environmental stress to the plant.
The maize Glb1 gene encodes globulin-1, a major embryo storage protein. (Kriz, et al. (1986) Plant Physiol. 82:1069-1075) Glb1 is expressed in the developing maize seed during embryo development. (Belanger, et al., (1989) Plant Physiol. 91:636-643) The promoter region of Glb1 has been identified, cloned, and introduced into tobacco plants by Agrobacterium-mediated transformation. (Liu, et al., (1996) Plant Cell Reports 16:158-162) The transformed plants demonstrate that the Glb1 promoter has desirable temporal and tissue specificity. However, the Glb1 promoter is positively regulated by abscisic acid (ABA). (Kriz, et al. (1990) Plant Physiol. 92:538-542; Paiva, et al., (1994) Planta 192:332-339) Levels of the plant hormone ABA are known to fluctuate under conditions of cold or desiccation. (Himmelbach, et al. (1998) Phil. Trans. R. Soc. Lond. 353:1439-1444) Thus, the activity of the Glb1 promoter can be differentially affected by environmental stress.
A maize cytokinin oxidase gene has been isolated and sequenced (GenBank entry AF044603). Cytokinin oxidase inactivates cytokinins, members of a class of plant hormones important in the control of cell division and in regulation of plant growth and structure. Elevated cytokinin levels are associated with the development of seeds in higher plants; exogenous cytokinin application has been shown to directly correlate with increased kernel yield in maize. Thus, control of the level of cytokinin oxidase has been suggested as a tool in improving grain yield. Manipulation of cytokinin oxidase activity has also been proposed as a means to achieve greater disease resistance or other improved plant characteristics. (See, WO 99/06571, herein incorporated by reference.)
However, a novel and heretofore undescribed utility of the isolated cytokinin oxidase gene is as a source of a promoter sequence with spatial and temporal specificity and which may be induced by cytokinins. A full-length promoter sequence of the isolated maize cytokinin oxidase gene, and functional fragments and variants thereof, and the use of such sequences with heterologous nucleotide sequences of interest, are described in the present invention. Unless otherwise specified, the notation “ckx1-2” in reference to the subject promoter includes SEQ ID NO: 1, SEQ ID NO: 4, and any functional fragments or variants thereof.